The present invention relates to an improvement in a thermal head for a thermal printer.
With the advent of computer technology and advances in the arts of data processing and/or data communication, requirements for increased speed of information handling have become more stringent. One known type of rapid printing is a high speed thermal printer, which has at least a thermal head and a printing paper, and operates on the principle that a thermal head, heated to a high temperature according to the pattern of a desired character to be printed, selectively changes the color of a thermal paper. A thermal printer has the advantage that it can print not only a predetermined pattern of characters, but also any pattern desired including pictures, Chinese characters and/or Arabian characters.
A thermal printer is a kind of a dot printer which composes the pattern to be printed with a plurality of dots, and a thermal head has a plurality of heat cells arranged, for instance, in a straight line for printing these dots. As the thermal paper moves in a direction perpendicular to said straight line of heat cells; said heat cells are selectively heated, thus the color of the thermal paper is selectively changed. Thus the desired pattern is printed on the thermal paper.
Some prior thermal heads are U.S. application Ser. No. 672,131 (filed Oct. 2, 1967), and U.S. Pat. No. 3,598,956. First, the features of those prior arts will be explained.
FIG. 1 shows a plane view of the main body of a prior thermal head. In the figure, the reference numeral 10 is a dielectric support, 11 through 18 are square heaters, 21 through 28 are lead lines each of which is connected to its corresponding heater and 30 is a common lead line connected to all of the heaters 11-18. The heaters 11-18 are a thin film of Ta.sub.2 N, NiCr, Ta or cermets, and the lead lines 21-28 are conductive layer made of Au, Ag or Cu. On the heater and the vicinity of the same, a pair of layers (not shown) for preventing the oxidation of the heater and for preventing the wear of the heater due to friction with the thermal paper are provided. Usually, said pair of layers are laminated in thin films on the heaters. Those layers are called protection layers.
FIG. 2 is a plane view of the main body of another prior thermal head. In the figure, the reference numerals 111, 112 and 113 are heaters made of Ta.sub.2 N, NiCr or Ta, all three of which operate to print a single dot. 121, 122 and 123 are also heaters and operate to print a single dot. Also the groups (131, 132, 133), (141, 142, 143), (151, 152, 153) and (161, 162, 163) are heaters and each group operates to print a single dot. The reference numerals 21 through 26, and 41 through 46 are lead lines each of which is connected to its corresponding heater for applying the electric power to the heater. 511, 512, 521, 522, - - - , 562 are conductors connecting the heater lines, and 611, 612, 621, 622, - - - , 662 are electrodes connecting the heaters and the lead lines. The protection layers are not shown in FIG. 2 for the sake of simplicity. The thermal paper moves in the direction of the arrow A in FIGS. 1 and 2.
FIG. 3 is a cross-sectional view at a-a' in FIG. 1, which also shows the protection layers 61 and 62. The protection layer 61 is for preventing the oxidation of the heaters and the protection layer 62 is for reducing the wear of the heaters due to friction with the thermal paper. The reference numeral 7 is thermal paper and 8 is a roller for moving the thermal paper 7 and pushing the thermal paper 7 into contact with the heaters.
However, the prior thermal heads in FIGS. 1 and 2 have the disadvantages described below due to the structures explained regarding FIGS. 1, 2 and 3.
(1) In manufacturing a thermal head the accurate positioning of a mask for a photo-etching process is necessary. First, the lead lines 21-28 and the common lead line 30 are manufactured through a photo-etching process, and next the heaters 11-18 are manufactured through an etching process. Accordingly, at least two etching processes are necessary, and the second process requires accurate mask positioning in relation to the first process. The requirement accurate positioning reduces the yield rate. The structure in FIG. 2 requires even more accuracy in the positioning of the mask, and further, it is very difficult to manufacture the heaters 111, 112, 113, 121, 122, - - - , 163, since those heaters are very thick.
(2) The life time of the prior thermal heads is short.
Generally speaking, the preferable resistance of a heater is in the range of 50 .OMEGA. to 300 .OMEGA.. If the resistance of a heater is lower or higher than that value, the electric current or the electric voltage reguired for generating the temperature for printing becomes too high, thus the external circuitry becomes complicated.
When the square heater in FIG. 1 is made of Ta.sub.2 N or NiCr the thickness of the heater for obtaining the above preferable resistance is only about 500 A, however, that thickness is not sufficient to provide the desirable long life. Further, the heater in FIG. 2 must be very thick in order to obtain said desirable resistance when the prior heater material is utilized, since the heater is folded and the total length of the same is rather long. According to the experiment, the thick film in FIG. 2 is rather difficult to make, and the film cannot be attached securely to the support, therefore, the yield rate is low.
(3) A prior thermal head has a cutaway portion at the contact area of the heater with thermal paper, and has steps S and S' as shown in FIG. 3, due to the difference of thickness and/or material between the heater 30, etc., and the lead line 23, 30, etc. Said cutaway causes the contact of the head with thermal paper to be incomplete, increasing the necessary power consumption since the thermal conductivity from the heater to paper is lessened, and the life time of the head is shortened since the heater must be over-energized due to incomplete contact. Further the dust from the paper is apt to be collected in the cutaway portion. The above situations are the same as those in FIG. 2.